Studies are continuing to examine the mechanisms by which oral actinomycetes adhere to other plaque bacteria and to mammalian cells. The hybridoma technique has been used to produce several monoclonal antibodies against surface fibrils (i.e. fimbriae or pili) on Actinomyces viscosus T14V. These antibodies have detected the lectin activity associated with adherence by cross-linking isolated fibrils to form soluble immune complexes with lactose-inhibitable agglutination activity for streptococci and neuraminidase-treated human erythrocytes. The antibodies cross-react only with human strains of A. viscosus and A. naeslundii having a surface lectin activity similar to that of A. viscosus T14V and also cross-link the isolated fibrils from these heterologus strains to reveal their lectin activity. These findings indicate that the presence of antigenically related fibrils on a wide range of actinomycete strains accounts for their similar lectin activity.